Older pools were typically plumbed with copper pipes, and copper turns out to be one of the most powerful algaecides ever discovered. The old pools we grew up with had a slow, constant leach of copper ions into the water, just enough to keep algae from ever getting a foothold. That's why algae problems weren't the headache then that they are now.
The green hair everyone blamed on chlorine
The most visible side effect of all that copper was something every blond kid of the era dealt with at the end of summer: green hair. Everyone blamed "too much chlorine," and they had it half right.
The chlorine wasn't the problem on its own. What was actually happening is that copper dissolved in the water was getting oxidized by the chlorine and falling out of solution, then binding to hair. (It bound to everyone's hair, but you could only see it on the blond ones, because oxidized copper is green.)
Copper does the same thing on a roof. A copper roof starts out shiny and golden, then oxidizes over years into the soft green patina you see on the Statue of Liberty. Same chemistry, different scale.
Modern plumbing, and what we lost
Today's pools are plumbed with PVC, usually 2 inches in diameter. PVC is cheaper, easier to install, and doesn't corrode. But it also doesn't leach anything antimicrobial into the water. Modern pools are essentially sterile from a plumbing standpoint, which means the algae control we got for free in the copper era now has to be done deliberately, every week, with chemistry.
The shift from copper to PVC also changed the hydraulics of the pool. That's the part most people don't think about, and it matters more than they realize.